Modulating system



Dec. 24, 1935. A' CQWAN 2,025,158

MODULATING SYSTEM Filed June 7, 1934 m/ve/vro/v F .A. COWAN PatentedDec. 24, 1935 UNITED STA MODULATING SYSTEM Frank Augustus Cowan, toAmerican Telepho New York, N. Y., assignor no and Telegraph Company, a.corporation of New York Application June 7, 1934, Serial No. 729,488

Claims.

This invention relates to modulating systems and more particularly tomodulating systems in which a plurality of undesired currents aresuppressed in the load circuit.

The main object of the invention is to'increase the simplicity andeconomy of modulating systems.

Another object is to improve the degree of suppression of the carrier'and of unwanted currents invarious portions of the system.

United States Patent No. 1,855,576, issued April 26, 1932 to C. R.Keith, shows a modulating system which provides for the suppression ofharmonies and certain other classes of modulation products in additionto the carrier and impressed signals in the load circuits, therebytransmitting substantially sidebands alone. Features of Keiths systeminclude the provision of three mutually conjugate circuit branches andthe adapt- 20 ability of the system for simultaneous use as a modulatorand a. demodulator. His arrangement utilizes balanced transformers,eight critically adjusted windingsin all being required, together withfour rectifiers which serve as modulating elements. In the present stateof the art, transformers are expensive, particularly if the windings arebalanced.

In accordance with this invention, I connect four rectifiers in a novelbridge arrangement with which only two transformers, including a totalof four balanced windings, are required to provide a circuit possessingall the above mentioned advantages. The number of components in mycircuit is reduced to a virtual minimum, with the result that greatsimplicity of design is effected and the cost is low.

The invention will be more fully understood from the following detaileddescription and the accompanying drawing of which,

Figs. 1 and 2 represent two different embodiments of the invention.

In Fig. 1, reference characters I, 2, 3 and Q indicate the ratio arms ofa bridge network, comprising rectifying elements all having their bestconductivity in one direction around the closed loop in which they arejoined. These rectifying elements may be of the copper oxide type or maybe diode vacuum tubes. A carrier current generator 5 is connected to onediagonal of the bridge through an input transformer 6 having a dividedsecondary winding. In the other diagonal of the bridge a load circuit 1is connected by means of an output transformer 8 having a dividedprimary winding. A telegraph key 9 and a battery Cl. 17917l) H] areconnected between the division points of the respective transformerwindings.

In the operation of the system of Fig. 1, when key 9 is open thealternating current from source 5 flows into the bridge but is nottransmitted to 5 the output circuit, assuming that the ratio arms areproperly balanced, as for example, when they comprise substantiallyidentical elements. During one-half cycle the current flows mainly inrectifiers l and 2 and during the other half cycle 10' in rectifiers 3and 4, but there is substantially no voltage induced across transformer8. When key 9 is closed, with the battery [0 poled as shown, a directcurrent flows from the battery, in two parallel circuits as follows: 1)through the upper part of the primary of transformer 8, rectifier 4, andthe upper part of the secondary of transformer 6; (2) through the lowerpart of the primary of transformer 8, rectifier 2, and the lower part ofthe secondary of transformer 6. At the same time the battery impresses ablocking potential across the two remaining rectiiiers. The conductingrectifiers 2 and 4 present a low impedance while the blocked rectifiersl and 3 present a high impedance to the alternating current from source5. Consequently, the bridge is unbalanced, permitting alternatingcurrent to pass from the source to the load. Moreover, the similarity ofthe rectifiers insures equal currents through the two portions of eachdivided winding. By using windings having two equal parts the currentstherein may be balanced out with with respect to their effects, uponthe, circuits connected to the respective transformers. In other words,currents from the battery do not induce voltages either across the loador across the generator 5, the key and battery acting merely as a meansto control the transmission of energy from the generator to the load.The arrangement may serve for example as a carrier 40 telegraphtransmitting system. The symmetry of the circuit insures that thegenerator shall not induce any appreciable voltage across the keyingcircuit. Usually the particular polarity ,of the battery is notmaterial.

. It is evident from the foregoing description that the system of Fig. 1may be adjusted in such man- I ner that it will possess three pairs ofmutually conjugate branches. The bridge diagonals are conjugate to eachother and each diagonal is in turn conjugate to the keying circuit. Itis apparent also that the three external circuits, namely thealternating current or carrier source, the control or keying circuit,and the load may be rearranged in any desired permutation as regards theparticular circuit branch each occupies.

Fig. 2 shows a telephone transmitter H connected to transformer 6 andthe generator 5 5 placed between the midpoints of the dividedtransformer windings. Otherwise, the arrangement is identical with thatin Fig. 1. This system operates as a radio or carrier modulator systemthe carrier wave being supplied by the 10 generator 5. Because of thebalance of the divided windings ,of transformers 6 and 8 thecarrierwave'is not transmitted either to the load .1 or the source II.By its biasing action on the rectifier elements, however, anintermittent flow 15 of the signal current through the system from thesource to the load is permitted. There is thus established in the load 1a modulated high frequency current corresponding to the signal currentsubjected to reversals at the carrier fre- 20 quency rate. This, as iswell known, represents a modulated carrier wave with the carriercomponent suppressed. For the best operation of the system the amplitudeof the carrier wave should be large compared with that of the signalwave. The. arrangement is especially useful in signal inversion schemesfor promoting, secrecy, such as in Kendall Patent 1,571,010 of January26, 1926. In these systems, it is usual to invert the frequencies of thesignal components. The inverted bandmay lie partly or wholly within thesame frequency range as the original signal. It i is then important tokeep any uninverted signal from reaching the load.

to suppress the carrier. The system of the in- 35 vention suppresses theuninverted signal, the

carrier, and all their harmonics, as well as certain of the combinationproducts incidental to 'modulation' Whatis claimed is: 1. A modulatingsystem comprising a rectifier bridge, an input transformer with-adivided secondary winding, said winding-being connected. across onediagonal of the bridge, an output transformer with a divided primarywinding, said 45 primary winding being connected across the otherFurther, it 'is desirable I diagonal of the bridge, and connections tothe respective division points, the rectifiers being so poled that saidconnections together with the two remaining transformer windingsconstitute three mutually conjugate circuit branches.

2. A modulating system comprising four rectifiers connected together ina closed loop each with its best conductivity in the same directionaround the loop, a network including said rectiiiers and having threemutually conjugate 10 branches, and a signal source, a carrier sourceand a load circuit connected respectively in the said three branches.

3. A modulating system comprising four rectifiers Joined inseries-aiding relation in a closed loop or bridge, an input transformerwith divided secondary winding connected across one diagonal of thebridge, a carrier source connected to the primary of said inputtransformer, an output transformer with divided-primary connected acrossthe other diagonal, a loadcircuit across the secondary of the outputtransformer, and a signal source bridged between the division points ofthe two transformers.

4. A modulating system comprising four rectiflers connected together inseries-aiding relation in a closed 100p or bridge, a signal sourceconnected in one diagonal of the bridge, a load circuit connected in theother diagonal, a carrier source, and means to connect said carrier 0source to the bridge in conjugate relation both to the signal source andto the load circuit.

5. A modulating system comprising four rectifiers joined inseries-aiding relation in a closed loop or bridge, an input transformerwith divided secondary winding connected across one diagonal of thebridge,- a signal source connected to the primary winding of the inputtransformer, an output transformer with divided primary windingconnected across the other'diagonal, a load circuit across the secondaryof the output transformer, and a carrier source bridged between thedivision points of the two transformers.

FRANK A. COWAN.

